The 'factor of safety' against base heave for a braced excavation in soft clay is checked using:
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Only slope stability formula of Bishop
Terzaghi''s base heave formula: FOS = NcSu/(γH+q) — critical in soft clays; governs excavation support design
Rankine earth pressure calculation
SPT N-value at excavation depth
Correct Answer
B. Terzaghi''s base heave formula: FOS = NcSu/(γH+q) — critical in soft clays; governs excavation support design
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Base heave in braced cuts (Terzaghi, 1943): FOS = NcSu/(γH+q) where Nc = bearing capacity factor (≈5.7 for B/L=1, strip), Su = undrained shear strength, γH = overburden, q = surcharge. B = width of excavation. FOS ≥ 1.5 typically. Failure: soil heaves into excavation from below → loss of strut preload → progressive collapse. Critical in soft clays with Su < γH/5.7.
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